Field of the Invention
This invention relates to light emitters and light transmission control materials to mask their appearance when inactive, and in particular to solid state emitters having light transmission control materials to mask their appearance when inactive but becoming transparent when the solid state emitter is active.
Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and extracted to the surrounding ambient from all transparent surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 512 is mounted on a reflective cup 513 by means of a solder bond or conductive epoxy. One or more wire bonds 511 connect the ohmic contacts of the LED chip 512 to leads 515A and/or 515B, which may be attached to or integral with the reflective cup 513. The reflective cup may be filled with an encapsulant material 516 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 514, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 512. While the reflective cup 513 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package, since it may be difficult to extract heat through the leads 515A, 515B.
A conventional LED package 520 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 520, one or more LED chips 522 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 523. A metal reflector 524 mounted on the submount 523 surrounds the LED chip(s) 522 and reflects light emitted by the LED chips 522 away from the package 520. The reflector 524 also provides mechanical protection to the LED chips 522. One or more wirebond connections 527 are made between ohmic contacts on the LED chips 522 and electrical traces 525A, 525B on the submount 523. The mounted LED chips 522 are then covered with an encapsulant 526, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 524 is typically attached to the carrier by means of a solder or epoxy bond.
LED chips, such as those found in the LED package 520 of FIG. 2 can be coated by conversion material comprising one or more phosphors, with the phosphors absorbing at least some of the LED light. The LED chip can emit a different wavelength of light such that it emits a combination of light from the LED and the phosphor. The LED chip(s) can be coated with a phosphor using many different methods, with one suitable method being described in U.S. patent application Ser. Nos. 11/656,759 and 11/899,790, both to Chitnis et al. and both entitled “Wafer Level Phosphor Coating Method and Devices Fabricated Utilizing Method”. Alternatively, the LEDs can be coated using other methods such as electrophoretic deposition (EPD), with a suitable EPD method described in U.S. patent application Ser. No. 11/473,089 to Tarsa et al. entitled “Close Loop Electrophoretic Deposition of Semiconductor Devices”.
In these arrangements the phosphor material is on or in close proximity to the LED epitaxial layers and in some instances comprises a conformal coat over the LED. In these arrangements, the phosphor material can be subjected to direct chip heating which can cause the phosphor material to heat. In other embodiments the phosphor is placed remote from the LED.
Lamps have been developed utilizing solid state light sources, such as LEDs, with conversion material on or around the LEDs. Such arrangements are disclosed in U.S. patent application Ser. No. 11/974,431 to Keller et al., entitled “Multiple Conversion Material Light Emitting Diode Package and Method of Fabricating Same.” Lamps have also been developed utilizing solid state light sources, such as LEDs, with a conversion material that is separated from or remote to the LEDs. Such arrangements are disclosed in U.S. Pat. No. 6,350,041 to Tarsa et al., entitled “High Output Radial Dispersing Lamp Using a Solid State Light Source.” The lamps described in this patent can comprise a solid state light source that transmits light through a separator to a disperser having a phosphor. The disperser can disperse the light in a desired pattern and/or changes its color by converting at least some of the light through a phosphor.
These conversion material coatings and dispersers in solid state emitters are visible to users when the emitter is inactive or not energized. For example, solid state light sources that generate white light by using phosphors to convert blue light have distinct yellow or yellow-orange appearances. In some applications these visible conversion materials can be considered unsightly by users and consumers, which may lead to adoption avoidance in the market. To mask this appearance, present technology relies on diffusive or semitransparent materials to modify the visibility of conversion materials in solid state light emitters. These solutions use materials which remain diffusive or semitransparent even when the light is energized resulting in significant transmission losses of light, 4% up to 50%, reducing performance of the light source.
In other applications, such as windows, privacy glass, oven windows, and artistic displays, reversibly light scattering materials have been used within surfaces or to coat surfaces to change their ability to reversibly scatter light or transparency. These coatings can be thermally or electrically triggered. Examples of these materials and their functions can be found in U.S. Pat. Nos. 6,362,303, 4,273,422, and 6,416,827.